Fuel cell systems are configured to: supply a hydrogen-containing gas (gas containing H2 gas) and an oxygen-containing gas to a fuel cell stack (hereinafter, simply referred to as a “fuel cell”); cause an electrochemical reaction between hydrogen and oxygen to progress; and extract resultant chemical energy as electrical energy to generate electric power. Fuel cell systems are capable of not only generating electric power with high efficiency, but also readily utilizing thermal energy that is generated during the power generation. Therefore, fuel cell systems are being developed as distributed power generation systems that make it possible to realize highly efficient energy utilization.
Generally speaking, it is often the case that a supply system for supplying the hydrogen-containing gas is not developed. Therefore, conventional fuel cell systems are provided with a hydrogen generation apparatus. A typical hydrogen generation apparatus generates a hydrogen-containing gas (reformed gas) by using, as a raw material, city gas containing natural gas as a main component, LPG, or the like supplied from an existing infrastructure. Accordingly, the hydrogen generation apparatus includes, for example, a desulfurizer configured to remove sulfur components from the raw material and a reformer configured to generate the hydrogen-containing gas by causing a reforming reaction between the raw material and steam at temperatures of 600 to 700° C. by using a Ru catalyst or Ni catalyst (see Patent Literature 1, for example).
Usually, the hydrogen-containing gas obtained from the reforming reaction contains carbon monoxide derived from the raw material. If the carbon monoxide concentration is high, the power generation performance of the fuel cell is degraded. Therefore, it is often the case that the hydrogen generation apparatus includes reactors such as a shift converter, a selective oxidizer, and a methanation remover in addition to the reformer. The shift converter includes a Cu—Zn based catalyst, and causes a shift reaction between carbon monoxide and steam to progress at temperatures of 200° C. to 350° C., thereby reducing carbon monoxide. The selective oxidizer selectively causes a carbon monoxide oxidation reaction at temperatures of 100° C. to 200° C., thereby further reducing carbon monoxide. The methanation remover selectively causes carbon monoxide methanation, thereby reducing carbon monoxide. The selective oxidizer and the methanation remover are also referred to as selective removers.
There are cases where oxygen is temporarily mixed into the raw material due to configurational features of the infrastructure. In this respect, there is proposed a method of pre-reforming an oxygen-containing process gas (e.g., natural gas, peak shaving gas, LPG, etc.) (see Patent Literature 2, for example).